1. Field of the Invention
The present invention relates to an optical scanner and an image forming apparatus and, more particularly, to an optical scanner that scans a subject to be scanned with a luminous flux and an image forming apparatus that includes the optical scanner.
2. Description of the Related Art
Optical scanners are used in image forming apparatuses, such as laser printers, optical plotters, digital copying machines, and facsimile machines. An optical scanner deflects a luminous flux emitted from a light source using an optical deflector and collects the deflected luminous flux on the subject to be scanned of an optical scanning system, thereby forming a spot of light on the subject to be scanned, and scans the subject to be scanned with the spot of light.
The subject to be scanned of the image forming apparatus is, for example, a photoconductive and photosensitive image carrier.
For example, a tandem-type color printer includes four image carriers arranged in parallel each other in the conveying direction of a recording sheet. Each image carrier has a cylindrical photosensitive element. An optical scanner used in the tandem-type color printer includes a plurality of light sources each of which corresponding to one of four colors (yellow, magenta, cyan, and black, in general). Luminous fluxes emitted from the light sources are deflected by one optical deflector. The deflected luminous fluxes pass through respective optical scanning systems and then scan respective image carriers, thereby forming latent images on the respective image carriers. Each latent image is then developed into a visible image with a developer of the corresponding color. Subsequently, the visible images are sequentially transferred onto a recording sheet in a superimposed manner and then fixed on the recording sheet. A color image is thus formed.
In recent years, the image forming apparatuses have been required to increase the speed of image forming and improve the quality of the images. One approach that achieves an increase in the speed of optical scanning is to increase the deflecting speed of optical deflectors, for example, to increase the rotating speed of polygon mirrors. This approach, however, brings problems of noise, heat, etc., due to high-speed rotation and any increase in the rotating speed has its limitations. As an alternative approach, to illuminate one image carrier with a plurality of luminous fluxes, thereby scanning a plurality of lines at the same time was devised.
This approach is realized using something known as a multi-beam light source, which includes a plurality of light-emitting elements.
In terms of improving the image quality, stabilizing the image density is especially required. For stabilizing the image density, it is necessary to evenly maintain the intensity of light that illuminates the photosensitive element. In order to maintain the intensity of light on the photosensitive drum at a constant level, in general, the intensities of the luminous fluxes emitted from light-emitting elements are set to be equal using an auto power control (APC); however, with an APC, it is difficult to correct variation in the intensity of light that occurs due to an optical scanning system.
As one of the causes of the variation in the intensity of light that occurs due to the optical scanning system, variation in the direction of polarization among the light-emitting elements is conceivable. If the direction of polarization varies, variation in the reflectance of the polygon mirror and the reflecting mirrors occurs, and variation in the transmittance of the scanning lenses and dustproof glasses also occurs, which results in an output image with uneven density.
Optical scanners that aim to suppress unevenness in the image density have been devised.
Japanese Patent Application Laid-open No. 2007-156248, for example, discloses an optical scanner that includes a plurality of light sources, an optical system, a deflector element, and a correcting optical element. The optical system includes a plurality of optical elements that cause light beams emitted from the light sources to form images on a surface of the object that is scanned. The deflector element deflects the light beams in the main-scanning direction, thereby scanning the surface of the object that is scanned. The correcting optical element is disposed on the output side of the deflector element and allows the deflected light beam to pass therethrough. In the above described optical scanner, correcting optical element is arranged so that the composite transmittance of the deflector element and the correcting optical elements are substantially equal at an arbitrary angle of deviation of the deflector element.
Japanese Patent Application Laid-open No. H6-148547 discloses an optical scanner that can perform shading correction. This optical scanner includes a plano-convex cylinder lens that causes a parallel luminous flux to form a linear image running in the main-scanning direction. The light-source-side surface of the cylinder lens has a plane surface. The input surface, i.e., the plane surface of the cylinder lens is coated with a birefringent oxide so that the cylinder lens can convert linearly polarized light into substantially circularly polarized light.
Widely used scanning lenses include a molded resin lens. The molded resin scanning lens makes the incident light to cause different birefringence depending on the position of incidence. This is because unequal temperature distribution or unequal stress distribution that occurs during the cool down of the high-temperature resin in a metal mold. When a luminous flux passes through the scanning lens, the direction of polarization of the luminous flux is changed in a different manner depending on the position from which the luminous flux is output, which causes variation in the reflectance of a reflecting mirror that is arranged downstream of the scanning lens.
When a light beam with the direction of polarization parallel to the sub-scanning direction (see FIG. 60) is input to the scanning lens and birefringence occurs, the polarized state of the light beam after passing through the scanning lens is changed as illustrated in FIG. 61. As is clear from FIG. 61, as the phase difference increases and the optical axis deviation increases, due to the birefringence, linearly polarized incident light has more polarization components parallel to the main-scanning direction and elliptically polarized light is output. The reflectance of p-polarized light reflected by the reflecting mirror is different from the reflectance of s-polarized light; therefore, if the polarization of the light beam varies depending on the image height, the deviation of the light use efficiency with respect to the image height increases.
The optical scanner disclosed in Japanese Patent Application Laid-open No. 2007-156248 only takes variation in the direction of polarization among the luminous fluxes emitted from the light sources into consideration, it does not take the birefringence of the scanning lens into consideration at all.
The optical scanner disclosed in Japanese Patent Application Laid-open No. H6-148547 does not take the effect of the birefringence of the scanning lens into consideration either.
With this configuration, even when a multi-beam light source and a molded plastic scanning lens are used, variation decreases in the intensity of light on the surface of the object to be scanned.
With this configuration, because an optical scanner according to the present invention is included, cost reduction is possible without decreasing the image quality.